fix(spec_runner): add --base-branch argument support (#428)

The frontend was passing --base-branch to spec_runner.py but the argument
wasn't defined, causing task creation to fail. This adds:

- --base-branch argument to spec_runner.py argparse
- Passing the argument to run.py when starting the build

Fixes task creation error: 'unrecognized arguments: --base-branch develop'

apps/backend/runners/spec_runner.py

@@ -192,6 +192,12 @@ Examples:
         action="store_true",
         help="Skip human review checkpoint and automatically approve spec for building",
     )
+    parser.add_argument(
+        "--base-branch",
+        type=str,
+        default=None,
+        help="Base branch for creating worktrees (default: auto-detect or current branch)",
+    )
 
     args = parser.parse_args()
 
@@ -318,6 +324,10 @@ Examples:
                 "--auto-continue",  # Non-interactive mode for chained execution
             ]
 
+            # Pass base branch if specified (for worktree creation)
+            if args.base_branch:
+                run_cmd.extend(["--base-branch", args.base_branch])
+
             # Note: Model configuration for subsequent phases (planning, coding, qa)
             # is read from task_metadata.json by run.py, so we don't pass it here.
             # This allows per-phase configuration when using Auto profile.